Could I just treat Ancient Greek adjectives like nouns?

Historically, the distinction between adjectives and nouns is a fairly recent one—not entrenched before the 18th century. The classical grammars referred to nominals, which included adjectives and nouns.

In addition, Greek, unlike English but like many other languages, can routinely use adjectives on their own without a noun. In fact, neuter adjectives were how Classical philosophers referred to abstractions: τὸ ἴσον, “the equal”, was how someone like Plato would refer to Equality.

There is only one way in which Ancient Greek adjectives differ morphologically from nouns. Feminine 1st declension nouns that are accented on the penult in the singular are accented on the penult in the plural: κινάρα ‘artichoke’, κινάραι ‘artichokes’. If a feminine adjective is accented on the penult in the singular, it is accented on the antepenult in the plural, by analogy with the masculine plural: δεύτερος, δευτέρα: δεύτεροι, δεύτεραι (not: δευτέραι).

With typical cluelessness, 19th century grammars say that ethnic names are an exception to the accentuation rule for nouns: Ῥοδία “Rhodian woman”, Ῥόδιαι “Rhodian women”, not the expected Ῥοδίαι. But of course, that isn’t an exception at all. That just shows you that to Ancient Greeks, Ῥόδιαι was not a noun but an adjective.

Are Ancient Greek ο declension masculine and α feminine the most perfect declensions?

Fascinating question.

I mean, adjectives and nouns have declensions, and so do articles and pronouns. If an article is going to have a declension, better it have a declension that’s strongly associated with genders (since gender signalling is a core function of adjectives), than the third declension, which did not differentiate masculines and feminines. The first declension is feminine, with distinct-looking masculines as a late development in proto-Greek; the second declension is masculine/neuter, with feminines as an occasional exception. So they did correlate with gender strongly.

Of course, most pronouns share the alternation of first and second declension that adjectives have anyway; and the article originated as a pronoun.

There’s probably some Indo-European behind why so many pronouns decline that way, patterning with adjectives and indicating gender overtly; then again, the first declension was a late development in Indo-European.

Perfect? Maybe. But careful with your direction of causation. The first declension appeared probably a millennium before the articles did; the articles had the form they did because the declensions were a good match for gender, and that was something that happened in adjectives way beforehand.

Third declension adjectives do exist, and third declension nouns definitely exist; they don’t differentiate masculine and feminine, and there aren’t as many pronouns in the third declension. (Of course, τίς, τί ‘who’ is hardly an obscure pronoun.) By some criteria, I guess that makes them less perfect. By some, they’re rather more perfect…